There are certain situations wherein a relatively wide band first signal containing digitally modulated symbols is desirably received in the presence of a relatively narrow band analog second signal occupying a frequency band that is in the wide frequency band of the first signal. For example, geosynchronous satellites containing transponders for C-band television emissions are typically spaced from each other by 2.degree. of orbital arc and emit 5 to 10 watts. In the early to mid 1980's, a 120.degree. K low noise block (LNB) downconverter was typical for terrestrial reception of analog television signals having video information frequency modulated on a C-band carrier emitted from the satellite. To bring the carrier to noise ratio (C/N) of the terrestrially received signals (typically about 11 dB) to acceptable levels, terrestrial antennas having parabolic reflector dishes with a 10' diameter were used. A 10' dish has a 3 dB beamwidth of 1.7.degree. (i.e. the 3 dB power points of the antenna power pattern are 0.850 from the dish boresight axis); the pattern has a first null 2.3.degree. from the dish boresight axis. Because the satellites are spaced 2.degree. apart, the 10' diameter dishes protect against adjacent satellites by receiving the signals from the adjacent satellites on the low gain side lobes of the antenna pattern. Typically, the antenna gain of the side lobes of a good parabolic reflector dish and feedhorn coupled with the reflector is 18 dB below the boresight gain. Thus a high gain antenna automatically protects the desired signal coming from the satellite along the antenna boresight axis from interference from other adjacent satellite signals. Based on this reasoning C-Band satellites are placed 2.degree. apart in the orbital arc.
Technology, however, has advanced to a point where, if only thermal noise were the dominating problem, the diameter of the reflector dish could be reduced. Consumer television receivers for receiving analog video signals are overdesigned to protect against interference from adjacent direct broadcast satellites. New technologies have led to development of much lower noise LNB's with noise temperatures as low as 25.degree. K. For example, a 7' dish driving a LNB downconverter having a 40.degree. K noise temperature has the same noise performance as a 10.degree. dish driving a 120.degree. K LNB downconverter.
However, satellites recently put into orbit or soon to be in orbit operate at maximum permissible power levels of 40 dB equivalent radiated power (EIRP) over most of the United States. When such a power level is factored into the C/N calculation it is found that a 4' diameter dish suffices for reception of analog C-Band f.m. video (C/N=10 dB). The 3 dB beamwidth of a 4' dish at C Band is 4.2.degree.; the first null is 5.8.degree. from boresight. Thus, the main lobe of an antenna having a 4' dish can span as many as four satellites that are 2.degree. apart. Generally speaking, satellites which are 4.degree. apart have identical frequency/polarization plans, leading to the possibility of significant interference from a satellite that is two positions away in the orbital arc from a satellite aligned with the dish boresight axis. This interference has not been conveniently filtered from the desired signal with a fixed parameter filter. This is true even though the spectrum of the unwanted signal is specified only as being in the bandwidth of the desired signal.
The United States Federal Communications Commission frequency/polarization plan requires satellite transponders having adjacent center frequencies on satellites 2.degree. apart to emit radiation having the same polarization. Since signals from these identically polarized transponders can and do overlap in frequency, signals from adjacent satellites also interfere with each other when they are both received by an antenna having a relatively wide beamwidth. Such an interfering signal cannot be conveniently filtered from the desired signal.
A conclusion that can be drawn from C-band transmission of analog video is that if thermal noise, rather than interference were the governing consideration, a smaller dish would be feasible. This conclusion has even greater validity for direct satellite broadcasting of digital video signals. Professional grade satellite C-band f.m. video receivers have an i.f. bandwidth of 30 MHz. In various scenarios the information rate of digital video relayed through a satellite transponder is 30 Mb/s. If the LNB, antenna size and satellite EIRP are equal for 30 Mb/s digital video and 30 MHz f.m. analog video signals, the carrier to noise ratio (C/N) for the analog case equals energy per bit (E.sub.b /N.sub.0) for the digital case; E.sub.b /N.sub.0 is a measure of noise density and thus is similar to signal to noise ratio (S/N) of an analog signal. However the E.sub.b /N.sub.0 required for satellite transmission of C-band f.m. video, assuming concatenated forward error correction coding consisting of a Reed Solomon coder and a high rate convolutional encoder, is 5.5 dB, not 11 dB, as is typical of the C/N for analog video. By applying the increased margin to dish diameter, a 4' dish can be reduced to a 2.4' dish, if thermal noise were the only consideration. Of course, the smaller dish provides even less spatial discrimination against adjacent satellites so interference from other satellites becomes more of a problem.
Small diameter dishes are important because they increase the number of potential terrestrial installations (particularly homes) of receivers for direct satellite broadcasting via digital modulation. A current K-band DBS (direct broadcast satellite) system is viable because it employs a one-and-a-half foot (i.e. about 0.5 meters) diameter dish that is unobtrusive, has low wind resistance and provides acceptable reception. A C-Band DBS (Direct Broadcast Satellite) service is expected to be more profitable and certainly viable in areas having small receiver dish diameters because fixed costs thereof are spread over a base of cable subscribers and residences having consumer television receivers responsive to signals derived from a down link having digital modulation.
A proposed solution to reduce interference is to increase the 2.degree. spacing between adjacent satellites. This does not seem to be feasible or likely. Hence, the interference problem between signals from adjacent satellites is most acute for C-band digital television. A technical solution is preferable to a political solution because it does not require coordination between satellite operators or compromises between the organizations involved.
An interference situation similar to that of small diameter dishes responsive to adjacent satellites can also exist for: (1) quadrature amplitude modulation (QAM) cable transmission of video wherein it is desirable for (a) a wide band digitally modulated signal containing television broadcast information and (b) a narrow band analog signal containing television broadcast information to occupy the same frequency band on the same cable conductors, and (2) terrestrial transmission through the air of (a) multilevel vestigial sideband high definition television information having a relatively wide bandwidth and (b) a narrow bandwidth analog television broadcast signal that is in the same frequency band as the high definition wide bandwidth information.